1. Field of the Invention
The present invention relates to a method of repair of turbines and parts thereof and is primarily concerned with the repair of stator blades sometimes known as nozzles or diaphragms, the purpose of which is to direct steam or gases onto the rotor blades.
In use of a turbine, stator blades situated at the high pressure end of the turbine are subject to wear caused primarily by particles carried by the steam, most of the wear occurring on the face which is directed towards the incoming steam. However, the reverse face of the stator blade is also subjected to wear due to the effect of steam or particles rebounding from the adjacent row of rotor blades onto the reverse face of the stator blades.
In the past where wear or damage of sufficient magnitude to impair the efficiency of a turbine had occurred, the stator blades were replaced which inevitably led to a very substantial expense.
It has been proposed to minimise such wear by providing on the trailing edges of the stator blades, the area most susceptible to damage, a Stellite insert. Due to the incompatibility of stellite with the parent material of the blade, usually a 12% chrome steel, a relatively soft weld material Inconel had to be used in order to ensure that the insert was properly secured to the stator blade and in view of the inclement conditions inside a turbine, the Inconel was subject to a much greater wear rate than the stator blade or of course the Stellite insert.
In parts of a turbine, e.g. the first set of blades of the high pressure stage or intermediate pressure stage, the stator blades are situated very close to each other making repair thereof extremely difficult. Whereas the worn trailing edge could in some cases be built up by several weld passes this meant all the new material put onto the blade was "cast" and in view of the number of passes needed the heat input to each stator blade was considerable which could lead to distortion which was subsequently very difficult, if not impossible to correct thus leading to uneven gaps between the stator blades and the chance of considerable inbuilt stress into the structure as a whole which again could lead to premature failure.